1. Field of the Invention
The present invention relates to methods for applying a varnish by inkjet printing to a substrate, e.g. to a printed image.
2. Description of the Related Art
A varnish is a transparent liquid applied to a surface for producing a glossy appearance. A varnish may also be designed to produce satin or semi-gloss sheens by the addition of “flatting” agents. These flatting agents, also often called matting agents, are particulate substances for scattering incident light rays on the varnished surface. The matting agent particles stand out from the varnish layer, invisible to the human eye. This requires the matting agent particles to have an average particle size of several microns to tens of microns. Such large particle sizes make reliable inkjet printing of a mat varnish impossible since the nozzles of an inkjet printhead generally have a nozzle diameter of about 30 μm or less. The major advantage of inkjet printing is that it allows variable data printing.
US 2006230965 (HEIDELBERGER DRUCKMASCHINEN) discloses an offset printing method wherein a transparent glossy varnish is coated on the entire printed surface of a print using a varnishing unit. In addition a mat varnish containing a high content of silicate matting agent can also be applied if a mat finish is desired. Even if large particle size of matting agents in a varnish would be feasible by inkjet, the use of two varnishes, a glossy and a mat varnish, for controlling the gloss of a print would make an inkjet printer more complex and expensive.
US 2010166975 (MGI) discloses in claims 12 and 13 an inkjet ink including an additive with a granulometry less than 50 μm, wherein the additive includes a flatting agent for obtaining a mat or satin varnish, and/or, flakes for obtaining a flaked varnish, and wherein the inkjet ink has a granulometry suited for passing through a nozzle when ink is deposited by an ink-jet on a printed substrate. There is no practical example disclosed of a mat varnish. However flatting agents having a particle size up to 50 μm implicitly require nozzles diameters in the inkjet print head for reliable inkjet printing being much larger than 50 μm, thereby also drastically reducing the print resolution of the mat varnish and the capability of controlling the gloss of a specific part of a printed image.
US 2006021535 (HEIDELBERGER DRUCKMASCHINEN) discloses a method for radiation curable printing and aftertreating a print, wherein the aftertreatment involves adjusting the level of gloss of the print by applying to the print particles matting the surface of the ink. The particles having a diameter of more than 5 μm are applied using a powdering device having powder nozzles.
Problems with gloss homogenity may be observed when the printing speed increases, such as e.g. in single pass inkjet printing. EP 1930169 A (AGFA GRAPHICS) discloses a UV-curable inkjet printing method using a first set of printing passes during which partial curing takes place, followed by a second set of passes during which no partial curing takes place for improving the gloss homogeneity.
Another method to produce a stripe-free, smooth and highly glossy surface is by using a fast flowing UV varnish. US 2006198964 (HEIDELBERGER DRUCKMASCHINEN) discloses a method for inkjet varnishing of a print, which comprises ejecting varnish drops by an inkjet printer onto a surface of the print, wherein the varnish drops are ejected in a screen pattern. In this way, the required amount of varnish is smaller than when a varnish layer is applied over the complete surface of the print. The screen may be an FM- or an AM-screen. This allows preventing disturbing line structures. Depending on the flow characteristics of the varnish that is applied, a glossy or a matt result may be obtained. To obtain a high gloss level, a UV varnish that has a low viscosity, and that thus flows easily, is used, while a UV varnish that has a high viscosity is used to obtain a matte surface. However, again two varnishes are required for controlling the gloss, including a varnish of higher viscosity which limits the printing speed. In industrial ink jet systems, there is a constant demand for increased printing speeds in combination with high image quality. The new print heads, designed for increasing printing speed, only operate with very low viscous inkjet inks and varnishes.
EP 2228230 A (XEROX) discloses a method of controlling gloss of an image through micro-patterning a radiation curable ink and/or overcoat, i.e. a varnish, by non-uniformly curing the ink and/or overcoat followed by flood curing. The non-uniformly curing of the ink and/or overcoat is achieved by applying radiation through a mesh mask, or by laser curing by means of rastering a continuous wave or pulsed laser. Including such curing means makes the inkjet printer more complex and expensive. The micro-pattern is imparted to the radiation curable ink and/or overcoat by providing micro-roughness to one or more portions of the radiation curable ink and/or overcoat. At least one gellant must be present in the overcoat which results in a solid-like overcoat composition that below 50° C. has a viscosity of about 103 to 107 mPa·s. This not only increases the energy consumption of the inkjet printer but also put some limitations on the type of substrates that can be printed upon due to their thermal sensitivity.
US 2010/0194837 A1 (RICOH) discloses an image recording method includes ejecting an ink to form an image on the surface of a recording layer of a recording medium; and then applying a glossiness imparting liquid on the surface of the recording medium.
GB 2423520 A (SUN CHEMICAL) discloses a sprayable energy-curable coating composition comprising an epoxide monomer or oligomer, a cationic photoinitiator, and a cyclic carbonate, wherein the cyclic carbonate is present in an amount of at least 7 wt. % based on the composition. The sprayable energy-curable coating composition is used as a varnisch formulation;
Micro-roughness refers to surfaces marked by irregularities and/or protuberances imperceptible to normal and unaided human sight and touch, which surfaces are capable of diffuse reflection of light.
Micro-patterning refers to an irregular (e.g. random) or regular patterning of one or more surfaces characterized by micro-roughness.
There is still a need for an improved method of inkjet varnishing a print which is capable of controlling the gloss of a print by using a single varnish of low viscosity that can be printed at high resolution, speed and reliability without the need of any flatting agents.